Hardest boss for me has to be Ganandorf on Zelda:TP for the Wii. I beat him first go, but the sword fight is so damn hard!

I'm sorry, you found that HARD!? LOL!!!! That was the poorest excuse for a final boss ever and ruined the end of TP, it was too damn easy not too hard!

Hardest last boss for me was Doom Dragon on Golden Sun the lost age, had the ability to totally nerf my Djinn! Having said that there are a lot of secret bosses in a lot of RPGs that are way harder than the last boss (for example Dullahan was way harder than doom dragon on GS)

Yeah, he's EFFED up. I even went through gamefaqs just to beat him. Even in KH2 I couldn't beat him... too damn fast...

Really? I beat him pretty easily in KH1 and 2 on Hard difficulty, I had to be lvl 99 and get all my forms, Valor, Final, Wisdom, and Master to lvl 7 each so I can max out all their abilities, tip of advice, pick up Defense on hard mode, not Attack, because I beat Sephiroth on Hard mode in KH2 without having to heal, no items, no Curaga, just smack him away, he hardly does any damage if you're level 99 and you chose Defense.

up to now, FFIII final boss... I had to beat his 4 guardians so I could beat the final boss "easily" (and by easily, I mean that it took me like 1hr to kill his guardians and half an hour for the final boss XD)